The present invention relates to a hydraulic device for controlling a delayed pressure change in a clutch or brake of a transmission shifted or otherwise operated under load.
A hydraulic device of the type to which the invention pertains may include a pump for the pressure medium operating essentially on the basis of a constant rate of fluid flow by volume; conduit means being connected to the pump, the clutch or brake; and control valve means for primary activation and de-activation of the clutch or brake. German printed patent application 25 12 778 discloses such a device and that device includes additionally a pressure control in the fluid path between the pump and the clutch or brake. A relief conduit from the clutch or brake is branched off a control conduit in the controller. Moreover, the controller includes a stepped or differential position for gradually closing this relief conduit when the clutch is energized to thereby obtain a gradual increase from zero pressure to operating pressure in the clutch. The piston must, however, open the relief conduit at least to the extent so that rapid action de-energization be permitted. It was found, however, that energizing and/or de-energizing a brake or clutch under load has a detrimental effect on any secondary user of pressure oil such as a circulating lubrication system. This is particularly so as the operating pressure of the secondary user is generally lower than the operating and energizing pressure of the primary user, e.g. the clutch.